Method of producing lustrous zinc



Patented Apr. 10, 1.951

METHOD OF PRODUCINGLUSTROUS z No Allan E. Chester, Highland Park, 111., and Bruno Leonelli, Highland Park, Mich., assignors to Poor & Company, Chicago, Ill., acorporation of Delaware No Drawing. Application August 27, 1948, Serial No. 46,562

This invention relates to lustrous zincplates or coatings and to a process for brightening and sealing zinc surfaces. V

It is well known that zinc surfaces tend'to turn white or grayish-white with age, a property sometimes referred to as white corrosion, which is apparently due to the formation of oxides and carbonates. Several processes have been devised for increasing the resistance of zinc surfaces to white corrosion. One of these involves dipping the article made of zinc or containing a zinc coating in a chromic acid bath. The resultant product, however, discolors and therefore leaves much to be desired.

Another method of treating zincsurfaces is the so-called bright dip wherein the zinc article or an article containing a zinc coating is dipped in a bath of chromic acid anhydride, sodium sulfate and nitric acid. This process hasthe disadvantage that the articles are hard to rinse and the bath has no substantial passivating effect .on the zinc. I

Improved brightness in articles containing an electroplated coating of zinc has been obtained by adding various'types of addition agents to the electroplating bath. This process is also used in conjunction with a bright dip of the type previously described.

Severalprocesses have been-described in the literature which are said to produce a bright zinc plate. One of these involves the treatment of a zinc plated article with a chromic acid bath containing a sulfate radical in a critical ratio-jollowed by treatment in a second step with a solution of an acid having a pH less than-2.68. Another process involves a treatment in one step with a chromic acid bath and in a second step with a caustic alkali solution having a normality between 4.25 and saturation. Still a third process involves a treatment in which glacial acetic acid is employed with chromic acid in afirstbath followed by a caustic dip in the second bath;

One of the objects of the present invention is to provide a new and improved method of treating zinc articles, or articles containing a surface coating of zinc, to produce a lustrous zinc surface possessing excellent brightness and unusual resistance against corrosion. v L

A further object of the invention is to produce new and improved corrosion resistant zinc sur-' face articles. Other objects will appear hereinafter.

In accomplishing these objects in accordance with the invention it has been found thatfnew and improved results in producing zinc coatings 10 Claims. (01. 148-621) of enhanced corrosion resistance canbe obtained by subjecting zinc or a zinc coated article successively to treatment with: v

(1) A bath containing chromic acid and of the type which forms a yellow-brown iridescent stain on the zinc; I (2) Rinsing the first bath from the zinc coating with wateror other suitable liquid; .and (3) While the coating is still wet,v that is, before it has been completely dried, applying thereto, preferably in the hot state degrees F.-18,0 degrees F.), a solution of sodium silicate. I

In the practice of the invention it has been foundthat the resultant zinc coating, or zinc coated article, has a much greater resistance to corrosion than zinc coatings which have been protected with secondary treatments. according to methods of the type described in the literature. Thus, in carrying out the present invention it has been demonstrated that-a zinc coated arti cle protected in accordance with this invention had a resistance to salt spray of 150-120 hours as compared with 100 hours where the article was protected by one of the heretofore known methods such as treatment in a chromic acid bath followed by treatment in a caustic alkali bath. It is believed that the added resistance to corrosion which is produced by the practicebf the present invention is due at least in part to the fact that the treatment in the chromic acid bath produces a gelatinous film whichlwhen subjected to treatment with the silicate before drying undergoes a chemical reaction to produce a different type film of superior corrosion resistance. i

The invention will be illustrated but is not limited by the following example. I

Q Example Zinc is plated onto sheet steel in a conventional 2 grams per liter of zinc" dust are stirred into,

the electrolyte, and the entire mixture' is filtered in order to remove tracesof heavy metals The plated article is'then rinsed and dried, or without drying is immersed in a bath having the following composition at room temperature ('75 degrees F.)

The zinc plated article is immersed in this bath from 1 to seconds and is then thoroughly rinsed and removed from the bath. It will be coated with a yellow-browri' iridescent stain, which apparently consists of chromic hydroxide (Cr(O'H)3) and various chromium chromates.

After the article has been removed from the first bath, above described, and thoroughly rinsed it is placed in a second bath having the following composition Sodium silicate (commercial or N grade)-1 to 12 ounces per gallon Tergitol Penetrant Oil-41.1% to 3% by weight of the bath.

In the foregoing example Tergitol Penetrant O8 is a, synthetic primary alcohol sulfate used as a wetting agent and .penetrant in concentrated solutions of alkalies, acids and salts. (See The Chemical and. Engineering Dictionary published by The Chemical Publishing Company of N. Y., Inc.). Instead of Tergitol O8, isopropyl naphthalene sulfonic acid sodium sulfate may be employed in approximately the same proportions, and other wetting agents may be used which have a wetting and penetrating action. The presence of the Wetting agent apparently speeds up or accelerates the treatment in the second bath. The N grade sodium silicate referred to in the above example is a commercial silicate of soda having the approximate formula NazO-3.3SiO2. The weight ratio of SiOz to NazO is approximately 3.22. This grade of sodium silicate contains about 8.90% NazO and about 28.7% SiOz. A hot bath is used in this treatment in order to avoid gelatinization of the sodium silicate. The action of the second bath on the product obtained from the. first bath is almost instantaneous, that is, itrequires only about 2 to 30 seconds.

Instead of sodium silicate other water soluble silicates and polysilicates may be employed to effect a similar result. The ratio of M to S102 in these silicates (M being sodium, potassium, lithium or other metal producing a soluble silicate) may vary within relatively wide limits. Thus, sodium metasilicate and/or sodium orthosilicate may be used.

In the first bath, the licorice powder is employed as a wetting agent to increase rinsability. Any other wetting agent which is stable in the presence of chromic acid may be employed. Another example of a wetting agent is quillaja extract. As will be understood, licorice powder is powdered licorice root. The per-sulfate constituent of the first bath also be varied somewhat and other per-sulfates, including potassium persulfate and sodium persulfate may be used.

The invention produces results that have never been obtained heretofore in the protection of zinc surfaces having unusual luster and corrosion resistance. The treatment described is particularly applicable to zinc plated articles and has been successfullyused in the treatmentof articles plated either in alkaline or acid baths.

The expression zincsurfaced article ifi Q31- ployed herein to cover any article having an exterior surface of zinc regardless of whether the article is made entirely of zinc or whether it merely contains a zinc coating. The expression zinc plated article is employed herein to describe a zinc coated article in which the zinc coating has been applied by an electroplating process.

The invention is hereby claimed as follows:

1. In a process of protecting zinc against corrosion, the steps which comprise treating a zinc surfaced article with a chromic acid solution to form thereon a yellow-brown iridescent stain, rinsing the excess chromic acid solution from the article so treated, and before the rinsed article is completely dry subjecting it to treatment with an aqueous solution of an alkaline metal silicate.

2. The process of producing a zinc surfaced article resistant to corrosion which comprises immersing the article in a bath containing chromic acid, rinsing the article, and before the rinsed article is dry immersing it in an alkaline silicate bath containing 1 ounce to 12 ounces per gallon of sodium silicate.

3. The process of producing a zinc surfaced article resistant to corrosion which comprises immersing the article in a bath containing chromic acid, rinsing the article, and before the rinsed article is dry immersing it in an alkaline silicate bath containing 1 ounce to 12 ounces per gallon of sodium silicate at a temperature within the range of degrees F. to degrees F.

4. In a process of protecting zinc against corrosion, the steps which comprise treating a zinc surfaced article with a chromic acid solution to form thereon a yellow-brown iridescent stain, rinsing the excess chromic acid solution from the article so treated, and before the rinsed article is completely dry subjecting it to treatment with an aqueous solution of an alkaline metal silicate containing a wetting agent.

5. In a process of protecting zinc against corrosion, the steps which comprise treating a zinc surfaced article with a chromic acid solution to form thereon a yellow-brown iridescent stain,

rinsing the excess chromic acid solution from the article so treated, and before the rinsed article is completely dry subjecting it to treatment with an aqueous solution of an alkaline metal silicate containing an alcohol sulfate which is stable in the bath and is capable of acting as a wetting and penetrating agent in amounts of at least 0.1 by weight of the bath and eifective to accelerate the action of said silicate on chromic acid treated zinc plated article.

6. The process of producing a zinc surfaced article resistant to corrosion which comprises immersing .a zinc surfaced article in a bath consisting essentially of 34 to 46 ounces per gallon of chromic acid to form on said article a yellowbrown iridescent stain, rinsing the excess chromic acid solution from the article, and then immersing the article in a second bath which consists essentially of an alkaline solution of a metal silicate.

'7. A process of producing a lustrous zinc surfaced article resistant to corrosion which comprises immersing a zinc surfaced article in a bath consisting essentially of 34 to 46 ounces per gallon of chromic acid, 3.4 to 4.6 ounces per gallon of ammonium persulfate, and a stable wetting agent in amounts efi'ective to increase rinsability, rinsing the article, and then immersing it in an a ka i e 1 16 31 32 bath consisting essentially of 1 ounce to 12 ounces per gallon of commercial sodium silicate in water.

8. A method as claimed in claim 7, in which the alkaline silicate bath contains a wetting and penetrating agent.

9. A zinc surfaced article containing a coating resulting from the treatment of the article with a chromic acid solution to form thereon a yellowbrown iridescent stain, rinsing the excess chromic acid solution from the article and before the rinsed article is completely dry treating it with an alkaline metal silicate in aqueous solution heated to a temperature in the range of 120 degrees F. to 180 degrees F.

10. A zinc surfaced article containing a coating of a gelatinous chromium compound film reacted in situ with a water soluble metal silicate applied in a wet state.

Y ALLAN E. CHESTER.

BRUNO LEONELLI.

6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

3. THE PROCESS OF PRODUCING A ZINC SURFACED ARTICLE RESISTANT TO CORROSION WHICH COMPRISES IMMERSING THE ARTICLE IN BATH CONTAINING CHROMIC ACID, RINSING THE ARTICLE, AND BEFORE THE RINSED ARTICLE IS DRY IMMERSING IT IN AN ALKALINE SILICATE BATH CONTAINING 1 OUNCE TO 12 OUNCES PER GALLON OF SODIUM SILICATE AT A TEMPERATURE WITHIN THE RANGE OF 120 DEGREES F. TO 180 DEGREES F. 